After reading up on induction generators I am inclined to think that this is what you have here.

In these animals the magnetic fields are created by the current flowing and vice versa. When it first starts spinning, only a small magnetic field is needed to get the current flow going, then after that the currents and field strengths build on one another.

Hi Tim I posted on MBY "Practical boat owner forum this morning under "Generator" and got sever semi informed answers1 You could always ask Downing and mills the question? I seem to remember something about exciting the coil and then it all will start to generate electricity But I`m not sure! should be some more answers tonight on MBY

A whole lot of answers. It seems to me that we can't answer the question without more detail of the DC generator.The typical car generatr is often called an alternator because indeed it generates AC which is rectified to DC output. The magnetic field required for all generators is produced by DC being fed via slip rings and brushes to the centre rotating component. They usually have a coil mounted with an axis in line with the shaft. The mag field is directed from the ends of the coil via pole pieces, steel bent around so that the N and S appear at the circumference of the component. There are multiple pole pieces ( at least 4)In this case there is no residual magnetic field (the pole pieces are soft iron) it requires some power from the battery to energise the field initially.

Old style care generators had the magnetic field produced by coils around the inside of the casing with pole pieces being just a stub to bring the field close to the rotating armature. Usually one coil divided in half (each side) so just one N and one S pole. These pole pieces did retain residual magnetism and relied on that residual magnetism to produce some output which then fed the field coils to reinforce the mag field.The current is actually generated by the rotating armature and the AC is converted to DC by the switching and collecting action of the commutator and brushes. Hence all output current goes through the brushes hence maintenance problems. This Dc generator is very similar to a DC motor so is the type used in a combined starter generator (Dyna start)

Then occasionally you will see generators DC or AC with or without rectifiers that rely on a permanent magnet to provide the magnetic field. Old bicycle light generators or wind generators. Because the field is permanent you lose the ability to control the power generated by controlling the field coil current. But you save wasted current.

Then there is the large aircraft generators. AC. They use a brushless generator (alternator in this case) Here the magnetic field is first produced by current from a battery ( or residual + from the output and controlled) in a coil around the outside of a small armature (rotor) this armature produces AC which is immediately rectified and fed to a larger rotating field coil. This rotates inside collecting coild on the outside which produce the output power. There are no slip rings commutator or brushes so with just bearings to support the rotor that may wear they are extremely reliable . This type involving 2 generators are usually only for high power ie many Kilowatts.

Bottom line does a gen require a residual magnetism some do some don't. The old car type used top have to generate some voltage from residual to get an output to reinforce the field so bringing it up to full voltage which then enabled a relay whci connected it to the battery. good luck olewill